Here is a list of projects whose documentation I found to be highly-detailed and well-written.
- The introductory tutorial of GNU Emacs (looks a lot better when run from within Emacs), is written in a very gentle and careful way; it's among the best tutorials I've seen. Same applies to the Emacs manual, which I'm tempted to call the best I've read yet. It really, really goes into painstaking detail, and seems to have been written by professional technical writers.
- CPython - I used to appreciate this more before I got closely involved, noticing warts here and there, and therefore taking it for granted.
- sqlite3 - quite extensive, especially for an embeddable DBMS